U.S. patent number 10,563,289 [Application Number 15/695,551] was granted by the patent office on 2020-02-18 for process for refining niobium-based ferroalloys.
This patent grant is currently assigned to Companhia Brasileira de Metalurgia e Mineracao. The grantee listed for this patent is Companhia Brasileira de Metalurgia e Mineracao. Invention is credited to Eduardo Augusto Ayroza Galvao Ribeiro, Clovis Antonio De Faria Sousa, Kleber A. Sernik.
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United States Patent |
10,563,289 |
Sernik , et al. |
February 18, 2020 |
Process for refining niobium-based ferroalloys
Abstract
Refined niobium-based ferroalloys are provided by removing lead
and other impurities therefrom by a process comprising charging
niobium ore concentrate and/or niobium oxide or a mixture of
niobium oxides to a metallothermic reaction chamber, admixing the
ore concentrate and/or niobium oxide with a reducing agent,
initiating a metallothermic reaction, under reduced pressure; and
allowing the reaction product to solidify and cool; crushing the
reaction product or crushing the niobium-based ferroalloy
previously reduced in open air, and charging the crushed product to
a melting crucible within a vacuum induction melting furnace,
lowering the pressure within the furnace to below 1 mbar, and
melting the crushed product while vaporizing the impurities
contained therein.
Inventors: |
Sernik; Kleber A. (Greensburg,
PA), De Faria Sousa; Clovis Antonio (Araxa, BR),
Ayroza Galvao Ribeiro; Eduardo Augusto (Sao Paulo,
BR) |
Applicant: |
Name |
City |
State |
Country |
Type |
Companhia Brasileira de Metalurgia e Mineracao |
Araxa |
N/A |
BR |
|
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Assignee: |
Companhia Brasileira de Metalurgia
e Mineracao (State Of Minas Gerais, BR)
|
Family
ID: |
63080227 |
Appl.
No.: |
15/695,551 |
Filed: |
September 5, 2017 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20190003012 A1 |
Jan 3, 2019 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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15638098 |
Jun 29, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C
33/00 (20130101); C22B 1/24 (20130101); C22B
34/24 (20130101); C22B 5/04 (20130101); C22B
4/06 (20130101); C22B 5/16 (20130101); C22C
35/005 (20130101); C22C 27/02 (20130101); C22B
9/04 (20130101) |
Current International
Class: |
C22B
34/24 (20060101); C22B 5/04 (20060101); C22C
27/02 (20060101); C22B 9/04 (20060101); C22B
5/16 (20060101); C22B 4/06 (20060101); C22B
1/24 (20060101); C22C 35/00 (20060101); C22C
33/00 (20060101) |
References Cited
[Referenced By]
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WO |
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Other References
Sattelberger et al, "Production of High Purity Niobium Mater
Alloys", Niobium Science & TechnologyProceedings of the
International Symposium Niobium 2001, Dec. 2-5, 2001, pp. 97-104.
cited by examiner .
Sattelberger, S., et al., "Production of High Purity Niobium Master
Alloys", Niobium, Science & Technology: Proceedings of the
International Symposium Niobium 2001, Orlando, Florida, U.S.A. Dec.
2-5, 2001, Published by Niobium 2001 Ltd., Bridgeville, PA, pp.
97-104; also available at:
https://www.gfe.com.fileadmin/user_upload/pdfs/Veroeffentlichungen/Master-
%20Alloys_HPNbMasteralloys.pdf. cited by applicant .
Mehra, O.K. et al., "Production of Ferro-Niobium", Transactions of
the Indian Institute of Metals, Calcutta, India, vol. 24, No. 1,
Feb. 28, 1971, pp. 66-70. cited by applicant .
Sousa, C. A. de Faria, "The Eveloution of FeNb Manufacutring",
Niobium, Science & Technology: Proceedings of the International
Symposium Niobium 2001, Orlando, Florida, U.S.A. Dec. 2-5, 2001,
Published by Niobium 2001 Ltd., Bridgeville, PA, pp. 89-96. cited
by applicant .
DeFuccio, R. Jr., et al., "A Semi-Continuous Autothermic Reduction
Process for the Production of Ferroniobium", INFACON 6, Proceedings
of the 6th International Ferraoalloy Congress, Cape Town, vol. 1,
Johannesburg, SAIMM, 1992, pp. 279-283. cited by applicant .
PCT International Search Report and Written Opinion dated Oct. 8,
2018, issued during the prosecution of PCT International Patent
Application No. PCT/IB2018/054824, 10 pages. cited by
applicant.
|
Primary Examiner: Koslow; C Melissa
Attorney, Agent or Firm: Locke Lord LLP Carroll; Alicia
J.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority to and is a
continuation-in-part application of U.S. patent application Ser.
No. 15/638,098 filed on Jun. 29, 2017, the entire contents of which
is incorporated herein by reference.
Claims
What is claimed is:
1. A process for producing low-lead niobium-based ferroalloys
comprising: charging niobium ore concentrate to a metallothermic
reaction chamber; admixing the ore concentrate with a reducing
agent; reducing the pressure in the reaction chamber to below
atmospheric pressure; initiating a metallothermic reaction; and
recovering a reaction product by allowing the reaction product to
solidify and cool.
2. The process as recited in claim 1, wherein an energy booster is
added to the resulting admixture prior to the metallothermic
reaction.
3. The process as recited in claim 1, wherein one or more elements
selected from the group consisting of chromium, molybdenum, cobalt,
iron, and nickel, oxides of any of the foregoing, and mixtures
thereof is added to the admixture prior to the metallothermic
reaction.
4. The process as recited in claim 1, wherein the metallothermic
reaction is conducted under a reduced pressure ranging from 100 to
300 mbar.
5. The process as recited in claim 1, wherein the niobium ore
concentrate is admixed with or replaced by Nb.sub.2O.sub.5,
Nb.sub.2O, NbO or an admixture thereof.
6. The process as recited in claim 1, further comprising: crushing
the reaction product; charging the crushed product to a melting
crucible within a vacuum induction melting furnace; lowering the
pressure within the furnace to below 1 mbar; applying power to the
system and melting said crushed product while vaporizing the
impurities contained therein, condensing the vaporized impurities
upon the exposed surface of a cooled condensing plate adapted to be
positioned above the crucible; removing said plate with the
condensed impurities thereon from the furnace under vacuum;
controllably oxidizing the condensed impurities; and recovering the
resulting alloy product having a lead content of 5 ppm or less.
7. The process as recited in claim 6, wherein after the pressure
within the furnace is lowered to below 1 mbar, the pressure within
the furnace is backfilled with an inert gas to bring pressure up to
about 100 mbar.
8. The process as recited in claim 7, wherein the condenser plate
is a metallic, water cooled condenser.
9. The process as recited in claim 8, wherein the condenser plate
is a copper condenser.
10. The process as recited in claim 6, wherein once the impurities
have been substantially removed from the melt, removing the
condensing plate with the condensed impurities thereon from the
furnace, and passing the condensing plate through an isolation
valve situated between the vacuum induction melting furnace and an
adjacent oxidizing chamber, while the furnace and oxidizing chamber
are under vacuum, closing the isolation valve, and admitting an
oxidizing agent or mixture into the oxidizing chamber in a
controlled manner to oxidize the condensed impurities, and
converting the impurities to a removable oxide dust.
11. The process as recited in claim 10 wherein once oxidation is
completed, a stream of inert gas is admitted to the oxidizing
chamber to dislodge and safely remove the oxide dust to an external
dust collector.
12. A process for producing low-lead niobium-based ferroalloys
comprising: crushing niobium ferroalloy previously having been
reduced in open air; charging the crushed product to a melting
crucible within a vacuum induction melting furnace; lowering the
pressure within the furnace to below 1 mbar; applying power to the
system and melting said crushed product while vaporizing impurities
contained therein; condensing the vaporized impurities upon the
exposed surface of a cooled condensing plate adapted to be
positioned above the crucible; removing said plate with the
condensed impurities thereon from the furnace under vacuum;
controllably oxidizing the condensed impurities; and recovering the
resulting alloy product having a lead content of 20 ppm or
less.
13. The process as recited in claim 12, further comprising, after
the pressure within the furnace is lowered to below 1 mbar,
backfilling the pressure within the furnace with an inert gas to
bring pressure up to about 100 mbar.
14. The process as recited in claim 13, wherein the condenser plate
is a metallic, water cooled condenser.
15. The process as recited in claim 14, wherein the condenser plate
is a copper condenser.
16. The process as recited in claim 12, further comprising, once
the impurities have been substantially removed from the melt,
removing the condensing plate with the condensed impurities thereon
from the furnace, and passing the condensing plate through an
isolation valve situated between the vacuum induction melting
furnace and an adjacent oxidizing chamber; while the furnace and
oxidizing chamber are under vacuum, closing the isolation valve,
and admitting an oxidizing agent or mixture into the oxidizing
chamber in a controlled manner to oxidize the condensed impurities;
and converting the impurities to a removable oxide dust.
17. The process as recited in claim 16, further comprising, once
oxidation is completed, admitting a stream of inert gas to the
condensing chamber to dislodge and safely remove the oxide dust to
an external dust collector.
18. The process as recited in claim 6, wherein the resulting alloy
product includes a lead content of 2 ppm or less.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to niobium-based ferroalloys and processes
for refining such alloys to safely eliminate impurities
therefrom.
2. Description of Related Art
The principal application for ferroniobium alloys (FeNb ISO 5453)
is the production of high strength low alloy steels, in which the
typical niobium content of the end product has a maximum of 0.10 wt
% Nb. Stainless steels, however, such as UNS S30940, S30741,
S31040, S31041, S31640, S33228, S34700, S34708, S34800, S34809, and
the like, generally contain from about 0.60 to 0.80 wt % Nb. And
nickel-based superalloys, such as Inconel 718, Inconel 625, Inconel
750, and the like, generally contain from about 0.70 to 5.50 wt %
Nb. When substantially higher niobium contents are implicated, the
contamination of the alloys with impurities such as lead may
severely harm the hot ductility of the resulting steel or alloy.
This ductility impairment can occur to such an extent that
scrapping of the material due to the formation of deep cracks
during the hot working operation, typically carried out on rolling
mills or forges, may become a recurrent problem. Moreover, the high
temperature properties, e.g., creep rupture and the like, can be
severely impaired, and in some cases, especially in superalloys,
the impurity content, e.g., lead and the like, may exceed the
typical specification limits.
It is therefore desirable to produce niobium-based ferroalloys
exhibiting a low content of elements harmful to the hot workability
and high temperature properties of the material that will receive
the niobium addition. Of these harmful elements, e.g., lead, tin,
bismuth, and the like, lead is one of the most harmful of them all,
in the quantities normally found in such ferroalloys.
A process commonly utilized to produce niobium-based ferroalloys
from niobium ore concentrate, which generally has a lead content of
about 50 ppm or less, is the basic chemical leaching process
followed by calcination. During the chemical leaching step, lead is
removed from the original ore concentrate, reacted with calcium
chloride and thereby precipitated as lead chloride. After the
concentrate goes through the leaching process and the lead chloride
is precipitated, the material is filtered to separate the mineral
from the liquid. The lead chloride that goes along with the ore
concentrate is vaporized in a calcination furnace. The effluent
gases are partly caught in the bag house of a dust collecting
system, after which the gaseous mass goes through a water scrubber.
This process however, has the potential of not being able to assure
that all of the lead removed from the concentrate will be totally
contained.
The present invention provides processes for removal of substantial
amounts of lead and other impurities from niobium-based ferroalloys
in a vacuum induction melting furnace.
SUMMARY OF THE INVENTION
This invention, in one embodiment, provides processes for producing
low-lead, i.e., less than 20 ppm lead, niobium-based ferroalloys by
means of processes which comprise: 1) charging niobium ore
concentrate obtained by a combination of physical and/or chemical
means which generally has a composition of about 60-70 wt. %
niobium, Fe.sub.2O.sub.3, SiO.sub.2, and TiO.sub.2, less than 5 wt.
% each, and BaO less than 25 wt. %, to a reactor suitable for
conducting a metallothermic reaction. The niobium ore concentrate
can be admixed with or replaced by niobium oxides, i.e.,
Nb.sub.2O.sub.5, Nb.sub.2O, NbO or admixtures thereof; wherein the
content of Nb.sub.2O.sub.5, Nb.sub.2O, NbO or admixtures thereof in
the overall ore concentrate/niobium oxide admixture can range from
0 to 100 wt. %; 2) The niobium ore concentrate and/or
Nb.sub.2O.sub.5 are further admixed with a reducing agent such as
aluminum, silicon, calcium, magnesium, and the like, and
preferably, with an energy booster such as alkali metal
perchlorates, peroxides, and the like; 3) other elements in their
metallic or oxide form, such as chromium, molybdenum, cobalt, iron,
and nickel, can also be added to the mixture, if desired. The
metallothermic reaction is then initiated in an environment of
reduced pressure, preferably, about 100 to 300 mbar, or, if
desired, at atmospheric pressure. The benefit of the reduced
pressure is to effect a reduction of any harmful impurities in the
admixture to a level below that normally achieved and, in the
specific case of lead, to a level below about 5 ppm when the
metallothermic reaction is conducted under reduced pressure and
coupled with further vacuum degassing effected in a vacuum
induction melting furnace as described hereinbelow; 4) the reaction
product is then solidified and cooled to permit safe handling
either under reduced pressure or under normal atmospheric pressure,
and 5) the solidified and cooled reaction product produced by the
above process of the present invention can then be crushed and
charged to a crucible placed within a vacuum induction melting
chamber situated within a vacuum induction melting furnace. After
the initial charging is completed, the chamber pressure is lowered
to below 1 mbar and then, if desired, the chamber can be backfilled
with an inert gas, such as argon, to about 100 mbar (to assist in
maintaining a leak-free furnace), and power is applied to melt the
load. During the meltdown of the charge, lead and other impurities,
e.g., tin--are further removed in the gaseous state. Prior to the
present invention, these vapors would condense and deposit on the
furnace walls, crucible coils, etc. and spontaneously ignite when
exposed to oxygen in the air, even in a rarified atmosphere.
In accordance with a further embodiment of the present invention,
the resulting metallothermic reaction product can be crushed and
charged to a crucible within a vacuum induction melting chamber,
the pressure within the chamber is reduced to below 1 mbar, and, if
desired, the chamber can be backfilled with an inert gas to about
100 mbar, and then power is applied to the system and said reaction
product is melted while vaporizing impurities contained therein,
the vaporized impurities are condensed upon the exposed surface of
a cooled, condensing plate adapted to be brought into the vacuum
induction melting chamber and positioned above the crucible and,
upon completion of the melting process, removing said plate from
the chamber with the condensed impurities thereon under vacuum,
controllably oxidizing the condensed impurities, and recovering the
reaction product having a lead content of 20 ppm or less.
BRIEF DESCRIPTION OF THE DRAWINGS
So that those skilled in the art to which the subject disclosure
appertains will readily understand how to make and use the devices
and methods of the subject disclosure without undue
experimentation, preferred embodiments thereof will be described in
detail herein below with reference to certain figures, wherein:
FIG. 1 is a partial side view of a schematic representation
illustrating one embodiment of the system for translating the
condensing plate between the crucible in the vacuum induction
melting chamber and an oxidizing chamber;
FIG. 2 is a partial side view of a schematic representation of one
embodiment of the present invention illustrating the relationship
between the crucible in the vacuum induction melting chamber and
the condenser; and
FIG. 3 is a partial cross section of the double vacuum seal
arrangement which, in one embodiment of the present invention, can
be used in the vacuum induction melting furnace to create an
essentially leak-free environment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will now be made to the drawings wherein like reference
numerals identify similar structural features or aspects of the
subject disclosure. For purposes of explanation and illustration,
and not limitation, a partial view of an exemplary embodiment of a
vacuum induction melting chamber in accordance with the disclosure
is shown in FIG. 1 and is designated generally by reference
character 10. Other embodiments of vacuum induction melting
chambers in accordance with the disclosure, or aspects thereof, are
provided in FIGS. 2-3, as will be described.
As shown in FIG. 1, a vacuum induction melting chamber 10 situated
within a vacuum induction melting furnace (not shown) is connected
to an adjacent oxidizing chamber 12 via an isolation valve 14
situated therebetween. A crucible 16 is seated within a rotatable
cradle 18 within the vacuum induction melting chamber 10. The
rotatable cradle 18 is adapted to tilt the crucible 16 to enable
discharge of the molten metal upon completion of the melting
operation and by back tilting the crucible, during the melting
operation, increasing the surface area of the melt thereby
increasing the efficiency of the removal of the vaporized
impurities therein. Back tilting of the crucible also avoids
bridging at the top of the charge. Bridging is a safety hazard
which can cause explosions. A condensing plate 20 is situated above
the refractory crucible 16 and is adapted to translate into and out
of the vacuum induction melting chamber 10 passing through the
isolation valve 14 and into the oxidizing chamber 12. The
condensing plate 20, preferably a water-cooled metallic condenser
made of copper or stainless steel, is affixed, in one embodiment,
to a carriage assembly 22 which permits the condensing plate 20 to
translate between the vacuum induction melting chamber 10, passing
through the isolation valve 14, to the oxidizing chamber 12. The
carriage assembly 22 effects translation of the condensing plate 20
by way of a hydraulically driven piston, a revolving screw-driven
arrangement, or the like.
When the condensing plate 20 is within the chamber 10, it is
positioned spaced above the refractory crucible 16. Means 24 are
provided for attaching the condensing plate 20 to the carriage
assembly 22 and to permit ingress and egress of coolant to
condensing plate 20.
The isolation valve 14 which connects the chamber 10 and the
oxidizing chamber 12 permits the condenser 20 to pass therethrough
while providing means for maintaining a vacuum in both the chamber
10 and the oxidizing chamber 12 and yet permitting the furnace and
the condensing chamber to operate independently of each other to
permit discharge of the melt from the furnace and controlled
oxidation of the impurities condensed on the condenser when the
condenser is in the oxidizing chamber.
In operation, niobium ore concentrate, in powder or granular form,
e.g., generally less than about 2 mm. thick, is optionally mixed
with or replaced by niobium oxide and further admixed with a
reducing agent such as aluminum and an energy booster such as
potassium perchlorate. Other metals or metallic oxides can also be
added to the mixture such as nickel, chromium, molybdenum, cobalt,
iron, and/or their oxides. The resulting mixture is charged to a
metallothermic reactor which optionally can be placed in a vacuum
chamber. In a preferred embodiment, the charged metallothermic
reactor is placed within a vacuum chamber enabling the production
of higher quality niobium-based ferroalloys. The metallothermic
reaction is ignited, preferably under reduced pressure. Upon
completion of the reaction, the resulting alloy is allowed to
solidify and cool to a point where it can be safely handled. The
resulting alloy is discharged from the reactor and crushed and then
charged to the melting crucible 16 within the vacuum induction
melting chamber 10. If desired, rather than employ the alloy
resulting from the metallothermic reaction described herein, the
alloy resulting from conventional reduction of niobium ore
concentrate in open air can be employed instead. Once the alloy,
regardless of how produced, is charged to melting crucible 16, the
condenser 20 is translated to a position above the melting crucible
16 within the vacuum induction melting chamber 10. Water-cooling of
the condenser is initiated by circulating cold water or other
coolant through the condenser. The pressure within the vacuum
induction melting chamber 10 and the adjacent oxidizing chamber 12
is lowered to below 1 mbar. An inert gas can be introduced, if
desired, to backfill the chamber and the adjacent oxidizing chamber
to a pressure up to about 100 mbar, and the power is applied to
melt the load.
As shown in FIG. 2, the refractory crucible 16 is adapted to be
rotated off its vertical axis 26 or back tilted along with the
condenser 20. In this manner, the exposed surface area of the
resulting melt is increased, enhancing removal of the volatile
impurities and bridging at the top of the charge is prevented. The
volatile elements, including lead, are quickly and preferentially
condensed on the surface of the cooled condenser 20 rather than
contaminating the furnace interior. Upon completion of the melting
process, the condenser 20, with the condensed impurities thereon,
is withdrawn from the melting chamber and translates into the
adjacent oxidizing chamber 12 through isolation valve 14, while
maintaining reduced pressure throughout the entire system. Once the
condenser is withdrawn into the adjacent oxidizing chamber, the
isolation valve 14 is closed, an oxidizing gas such as air, oxygen,
or a mixture of oxygen and an inert gas such as argon, or the like,
is gradually introduced at a controlled rate into oxidizing chamber
12 promoting oxidation of the condensed impurities in a manner that
does not present a safety hazard to the environment and personnel.
In order to ensure that no condensed metallic impurities will be
oxidized prematurely when the condenser is withdrawn from the
melting crucible and passed from the vacuum induction melting
chamber into the adjacent oxidizing chamber 12, no air is permitted
to enter the oxidizing chamber 12, until after the isolation valve
is closed. It is considered preferable that the vacuum induction
melting furnace be constructed in an essentially leak-free
configuration as discussed below.
As shown in FIG. 3, in order to insure an essentially leak-free
configuration, all sealing surfaces of the vacuum induction melting
furnace 28, such as sealing surfaces 30 and 32 of the access port
to the vacuum induction melting furnace 28 are equipped with a
double vacuum sealing arrangement about the perimeter of the
furnace between the lid 34 and body 36 of the furnace 28. Two
compressible sealing elements 38 and 38' are compressed along the
perimeter between the lid 34 and the body 36 of the furnace 28.
When a vacuum is being drawn in the furnace, the space 40 between
sealing elements 38 and 38' is also independently evacuated via
conduit 42 connected to a vacuum pump (not shown) to a pressure
(P.sub.2) lower than the pressure (P.sub.1) inside the furnace. In
this manner, a reduced pressure environment is maintained within
the furnace throughout the process essentially preventing the
infiltration of the external atmosphere and also serving as an
early warning system for a potential leak hazard at the interface
between the lid 34 and body 36 of the vacuum induction melting
furnace 28.
If desired, the resulting niobium-based ferroalloy may be retained
for an additional period of time under reduced pressure in the
vacuum induction melting furnace to achieve further refining. The
final lead content of the ultimate niobium-based ferroalloy can be
reduced in this fashion to 0.0020 wt. % or lower, i.e., 20 ppm or
lower, if the metallothermic reaction is conducted at atmospheric
pressure, and, to less than 5 ppm, if the reaction is conducted
under reduced pressure.
Once the controlled oxidation of the condensed impurities is
completed, the impurities, in the form of a dust of mixed oxides of
the metallic impurities, can be removed from the adjacent oxidizing
chamber 12 and collected in dust collector 44 for safe
disposal.
EXAMPLES
Example 1--Production of Refined Ferroniobium Alloy
The following example illustrates the effectiveness of the present
invention in reducing the lead content of ferroniobium alloys to 20
ppm or less.
Ferroniobium, obtained by an aluminothermic reduction reaction and
having a lead content of 0.075 wt %, is charged to the melting
crucible of an essentially leak proof vacuum induction melting
chamber. A copper, water-cooled condenser is situated within the
vacuum induction melting furnace and is adapted to translate
between the furnace and an adjacent oxidizing chamber through an
isolation valve forming the interface between the furnace and the
oxidizing chamber, whereby the condenser can be positioned over the
melting crucible. The condenser is also adapted to rotate with the
melting crucible while maintaining the reduced pressure throughout
the system. Once the ferroniobium alloy is charged to the melting
crucible, the condenser is moved over to a position above the
melting crucible, water cooling of the condenser is initiated, the
chamber pressure in the vacuum induction melting furnace is lowered
to 0.1 mbar and then backfilled with argon to 100 mbar. Power is
then applied to the induction coils to melt the charge. The
temperature within the furnace is maintained at 1600.degree. C. The
furnace, with the condenser spaced above the crucible, can be
tilted, if desired, to maximize the surface area of the melt.
Periodically, samples are withdrawn from the system and analyzed
for lead content. The following table summarizes the results.
TABLE-US-00001 Time After Complete Temperature Meltdown of Charge
(.degree. C.) Pb % wt Original Material -- 0.075 0.33 hr 1600 0.016
1 hr 1600 0.003 2 hr 1600 0.001
The vacuum induction melting procedure results up to 99 wt %
removal of lead and other impurities from the ferroniobium alloy.
The vaporized lead and other impurities condense on the exposed
surface of the cooled copper condenser. While maintaining the
reduced pressure, the condenser is retracted from the crucible and
passed through the isolation valve into the adjacent oxidizing
chamber. Once the isolation valve is closed, the furnace can be
tapped and the melt discharged from the crucible into
solidification molds. Then the isolation valve 14 is closed and, in
a controlled manner, oxygen or a mixture of oxygen and an inert gas
is permitted to enter the oxidizing chamber effecting oxidation of
the lead and other impurities without causing serious fire or
explosion. A powdery dust of metallic oxides of the impurities
resides within the chamber, whereupon, a stream of inert gas, e.g.,
argon or the like, is admitted to the chamber under the influence
of the reduced pressure in the system, effectively dislodging and
removing the dust to collection means such as a collection bag or
container, without creating a safety hazard.
Example 2--Production of Refined Niobium-Based Ferroalloy
Containing Nickel
The following example illustrates the effectiveness of the present
invention in reducing the lead content of niobium-based alloys
containing nickel to 20 ppm or less.
A blend of ferroniobium (ISO 5453) together with NiNb is charged to
a melting crucible sealed within a vacuum induction melting furnace
made essentially leak proof in the manner shown in FIG. 3. As in
Example 1, a copper, water-cooled condenser translates from an
adjacent oxidizing chamber through an isolation valve and is
positioned over the melting crucible. The condenser is also adapted
to translate from its position over the melting crucible and to
pass through the isolation valve back into the adjacent oxidizing
chamber, while maintaining the reduced pressure throughout the
system. Once the ferroniobium alloy together with NiNb is charged
to the melting crucible, the condenser is positioned over the
melting crucible, water cooling of the condenser is initiated, the
chamber pressure in the vacuum induction melting furnace and the
adjacent oxidizing chamber is lowered to 0.1 mbar and backfilled
with argon to 100 mbar, and then the power is applied to the
induction coils to melt the charge. The temperature within the
furnace is maintained at 1,600.degree. C. Periodically, samples are
withdrawn from the system and analyzed for lead content. The
following table summarizes the results.
TABLE-US-00002 Time After Complete Temperature Meltdown of Charge
(.degree. C.) Pb % wt Original Material 1,600 0.075 0.33 hr 1,600
0.016 1 hr 1,600 0.003 2 hr 1,600 0.001
The vacuum induction melting procedure results in extensive removal
of lead from the resulting ferroniobium nickel alloy. The vaporized
lead and other impurities preferentially condense on the exposed
surface of the cooled copper condenser. While maintaining the
reduced pressure, the condenser is retracted from its position over
the crucible and passed through the isolation valve into the
adjacent oxidizing chamber. Once the isolation valve is closed, the
charge is tapped into solidification molds and then the vacuum can
be broken and the molds withdrawn from the furnace. Then, the
isolation valve is closed and, in a controlled manner, an oxidizing
mixture of argon and oxygen is permitted to enter the adjacent
oxidizing chamber effecting oxidation of the lead and other
impurities without causing serious fire or explosion. A powdery
dust of metallic oxides of the impurities resides within the
chamber, whereupon, a stream of inert gas, e.g., argon or the like,
is admitted to the chamber with the aid of the reduced pressure in
the system, effectively dislodging and removing the dust to
collection means such as a collection bag or container, without
creating a safety hazard.
In the same manner, the nickel can be replaced with iron, chromium,
cobalt, and the like to obtain the corresponding niobium-based
ferroalloys containing the foregoing elements or mixtures
thereof.
Example 3--Production of Ferroniobium Nickel Alloy
A mixture of Nb-ore concentrate, Nb.sub.2O.sub.5, nickel,
KClO.sub.4 energy booster, and metallic aluminum powder are charged
to a reactor in a vacuum chamber. A vacuum is drawn to about 100
mbar and an aluminothermic reaction is initiated. After the
reaction is completed, the material is allowed to solidify and cool
to a temperature compatible with safe handling. The pressure is
then allowed to return to atmospheric pressure and the crucible is
removed from the vacuum chamber. The resulting ferroniobium nickel
alloy is removed from the crucible, cleaned and crushed.
The resulting ferroniobium-nickel alloy is then charged to a
melting crucible in a vacuum induction melting furnace and melted
therein as in Example 1 to remove substantially all the remaining
lead and other impurities. In this manner, the lead content in the
resulting alloy is less than 5 ppm.
Example 4--Production of Ferroniobium Nickel Alloy
A mixture of ferroniobium, refined niobium oxide, KClO.sub.4
temperature booster, nickel, and aluminum powder is charged to a
crucible in a vacuum chamber. A vacuum is drawn and an
aluminothermic reaction is initiated. Upon completion of the
reaction, the resulting ferroniobium nickel alloy is recovered,
cleaned and charged to a vacuum induction melting furnace and
remelted therein as in Example 1 to remove substantially all of the
remaining lead and other impurities.
* * * * *
References